fix(desktop): seed timeline virtualization row heights

[wesbillman]

Summary

• seed Virtua with per-row timeline height estimates while keeping measured heights authoritative
• memoize estimate arrays so ordinary scroll does not recompute them
• paint estimated rows immediately outside prepend compensation, while preserving Virtua's visibility safety during prepend
• keep the row shell component type stable so prepends do not remount mounted messages

Safety

• adds an E2E call-count gate proving five ordinary scroll moves do not recompute row estimates
• adds a prepend DOM-identity gate proving the mounted anchor node survives history insertion
• keeps image placeholders/blurhash polish out of this PR

Validation

After rebasing onto origin/main:

• pnpm check (passes; 2 pre-existing unused-code warnings)
• cd desktop && pnpm test — 2,886 passed
• cd desktop && pnpm build
• cd desktop && pnpm exec playwright test --project=smoke tests/e2e/timeline-no-shift.spec.ts tests/e2e/scroll-history.spec.ts --repeat-each=3 — 66 passed

Manual verification

Chromium automation covers estimate memoization, mounted coverage, anchor stability, and prepend node identity. The transient WKWebView fast-scroll blank still needs a final desktop eyeball because Chromium cannot directly reproduce that compositor behavior.